Effect of methylmercury on primary photosynthesis processes in green microalgae Chlamydomonas reinhardtii

The sensitivity of green microalgae Chlamydomonas reinhardtii to methylmercury chloride (MeHg) and chloride mercury (HgCl2) was evaluated by measuring chlorophyll fluorescence parameters by the pulse-amplitude-modulation (PAM) fluorometry. It was shown that MeHg at concentrations above 1 microM decreased the Fv/Fm ratio, which characterizes the maximal efficiency of energy utilization in photosystem II. The degree of inhibition depended on the time of treatment and was always higher under illumination conditions (50 microE.m-2.s-1) than under dark conditions. A similar regularity was observed for the delta F/Fm' ratio, which characterizes the real efficiency of energy storage at the given intensity of the photosynthesis-exciting light. Incubation with 5 microM HgCl2 for 5 h did not affect both ratios. The decrease in Fm at constant F0 as well as changes in the fast fluorescence kinetics after MeHg treatment of algae cells indicated the damage on the donor side of photosystem II and the damage of the electron transfer from QA to QB. The reduction of photochemical fluorescence quenching (qN) under MeHg treatment is also evidence of the increase in the fraction of closed reaction centers (QA-). At the same time, increase in the steady-state level of P700 photooxidation indicated a disturbance of electron transfer between photosystems. The present study demonstrates that methylmercury treatment damaged the photosynthetic electron transfer chain at several sites. The inhibitory effect of methylmercury is much stronger than the effect of mercury chloride on photosynthetic processes.